1. Field of the Invention
The present invention relates generally to suppression of stimulated Brillouin scattering occurring in an optical fiber transmission line, and more particularly to an optical transmitter, an optical communication system, and an optical communication method allowing such suppression.
2. Description of the Related Art
In recent years, an optical signal with high power and less chirping has been attainable by using an optical amplifier and an external optical modulator. In transmitting such a high-power coherent optical signal through an optical fiber transmission line, there occurs stimulated Brillouin scattering (SBS) in the fiber, causing a degradation in receiving sensitivity. Accordingly, any countermeasures against SBS are desired.
Conventionally known is an optical transmitter such that a bias current for inducing laser oscillation is supplied to a laser diode, and a modulating current is superimposed on the bias current to thereby obtain a modulated optical signal.
Also conventionally known is an optical transmitter such that a constant or controlled bias current is supplied to a laser diode, and continuous wave (CW) light output from the laser diode is modulated by an external optical modulator.
Meanwhile, it has been proposed that an optical signal intensity-modulated, for example, is further frequency-modulated (or phase-modulated) in order to suppress SBS. The bandwidth (spectral line width) of the optical signal is extended by the frequency modulation, thereby increasing a threshold value on a nonlinear effect. As a result, SBS is suppressed.
For the frequency modulation of light output from a laser diode, it is effective to superimpose a low-frequency signal on a bias current to be supplied to the laser diode. However, the superimposition of the low-frequency signal on the bias current brings about not only the modulation of the frequency (wavelength) of the optical signal, but also the modulation of the intensity (power) of the optical signal. Such intensity modulation by the low-frequency signal degrades transmission characteristics of a main signal, causing a problem such that long-distance transmission becomes difficult, for example.